The present invention relates generally to cablemaking, and specifically to an apparatus and method for the tandem extrusion in a common head of primary insulation and insulation shielding onto an advancing shielded conductor without causing inward protrusion of hard particles from insulation shield compound into the primary insulation.
Thermoplastic and cross-linked polyethylene insulated power cables rated 5 through 69 KV generally consist of a conductor surrounded by an extruded semiconducting shield material and extruded primary insulation which is in turn surrounded by an extruded semiconductive shield material. The insulation shielding may be either thermoplastic or thermosetting and is applied directly over the insulation. For cables rated 5 through 35 KV, the shielding must be removable without damaging the insulation and when removed must leave no conductive material on the insulation which cannot be readily removed. In order to economically produce such a cable, wire and cable manufacturers have attempted to develop a practical method of tandem extrusion in a common crosshead of at least the primary insulation material and the insulation shielding material, U.S. Pat. No. 4,081,232 illustrates extrusion tooling for such a method. The apparatus disclosed and claimed in U.S. Pat. No. 4,081,232 generally consists of an extruder die assembly for extruding a plurality of concentric layers of polymeric materials over an advancing core wire. The extruder die assembly comprises an outer die block means having a first passage therethrough with a first longitudinal axis and including a first internal engaging portion. The outer die block means of U.S. Pat. No. 4,081,232 includes an annular guiding shoulder formed with an inwardly directed first abutment surface, an intermediate passage portion concentric to the first axis and terminating in a first plane transverse to the first axis and a generally smooth decreasingly tapered surface extending from the abutment surface to the intermediate passage. The extruder die assembly further comprises a guiding die means having a second passage therethrough with its axis coincident with the first axis, first flange means which cooperates with the first internal engaging portion and having an aligning portion cooperating with the first abutment, a multiplicity of ports formed in the flange for permitting flow of a first coating material therethrough and a cylindrical tubing tip concentrically disposed within the intermediate passage for forming a first annular and concentric material passageway and terminating in the first plane. The guiding die means also includes a thin walled conical section interconnecting the tip and the first flange, a second internal engaging section formed internally of the first flange and a second generally annular guiding shoulder formed with an inwardly directed second abutment surface. An inner guiding means of the extruder die assembly defines a third passageway with its axis substantially coincident to the first axis and includes a third flange which has an external surface which engages the second internal engaging section and a multiplicity of spaced ports, a second alignment portion which functions in conjunction with the second abutment shoulder and a second tubing tip concentrically disposed within the first tip and terminating in the first plane for forming a second annular and concentric material passageway.
The shielding material applied by using such an apparatus is generally either a thermoplastic or cross-linkable polyethylene compound which is filled with a substantially high concentration of a conductive grade carbon black. Carbon black is a particulate form of generally elemental carbon which has many industrial uses. The particle size of electrically conductive grade carbon black of the type used to prepare semiconductive insulation shielding compounds ranges from about 0.04 to about 0.45 microns, however, carbon black particles have a tendency to agglomerate and therefore could form an agglomerate within the polymer matrix of a shielding compound many times larger than the individual particles and such agglomerates might even form hard protrusions which might project from the insulation surface of the insulation shield and into the primary insulation of a power cable of the type previously described. Such protrusions have generally extended up to 40 mils into the insulation and have usually lead to the formation of stress points within the insulation matrix and electrochemical and eventually premature failure of the power cable. The problem of hard particles of carbon black protruding from the insulation shield and piercing the primary insulation is especially acute when tandem extrusion in a common crosshead such as the apparatus disclosed in U.S. Pat. No. 4,081,232 is used to apply the insulation and insulation shielding to a power cable because both the insulation and insulation shielding are in the molten state when they are applied to the conductor and being similar polymers generally undergo some mingling at the insulation--insulation shielding interface thereby allowing hard particles to easily penetrate the soft insulation and cause the formation of such stress points. Evidence of this mingling process is easily seen when the insulation shielding is stripped from the insulation because conductive protrusions generally remain in the primary insulation.
An additional problem resulting from the tandem extrusion of insulation and insulation shielding in a common crosshead such as the apparatus of U.S. Pat. No. 4,081,232 is the poor stripability of the insulation shield applied to the insulation in this manner. The insulation shield material of 5 through 35 KV cable must be removable without damaging the insulation and when removed must leave no conductive material which cannot be readily removed. When insulation and insulation shielding are applied to an electrical power conductor using apparatus and methods of the type disclosed in U.S. Pat. No. 4,081,232 the insulation and insulation shielding come together in a molten state in a common extrusion plane, and being similar polymers the insulation and the insulation shield have a tendency to mingle and be bonded together at this interface thereby making it very difficult to strip the insulation shielding from the insulation in accordance with industry specifications.
Another problem associated with the production of insulated power cable rising apparatus similar to the apparatus of U.S. Pat. No. 4,081,232 is the fact that many times the conductor will be improperly centered with respect to the insulation or the insulation will be improperly centered with respect to the insulation shield and it is virtually impossible to properly center conductor, insulation and insulation shield from one common adjustment. If the cable is not properly centered, the insulation and insulation shield will not be concentric to the conductor and the cable will be useless.